Pivoting or oscillating torsional hinged mirrors provide very effective yet inexpensive replacements for spinning polygon shaped mirrors in printers and some types of displays. As will be appreciated by those skilled in the art, torsional hinged mirrors may be MEMS type mirrors etched from a silicon substrate using processes similar to those used in the manufacture of semiconductor devices. Earlier versions of torsional hinge mirrors for providing a raster type scan for printers and displays often operated at rotational speeds of about 3 KHz or less. Torsional hinged mirrors operating at 3 KHz or slower can be manufactured thick enough so that they do not demonstrate serious flatness problems with respect to the reflective surface. However, as the demand for higher print speeds and better resolution increased, flatness of the mirror reflective surface has now become a much more serious problem. As the mirror continuously flexes or bends back and forth during the continuous oscillations about the axis, the greatest deformation was at the tip or ends of the flexing mirror. Presently available mirrors have substantially reduced this problem by the use of a hinge plate that includes a center spine that extends along the long axis of the elliptical shaped mirror to each of the tips or ends of the mirror. Unfortunately, with greater rotational speeds and thinner and smaller mirrors, new flexing modes around the edges now affect the flatness of the mirror during operation.
More specifically, referring to FIGS. 12A and 12B, there is illustrated how the edges 20 and 22 of the mirror layer 24 flex. Central spine 26 formed as part of the hinge plate 28 extends almost to the tip 30 of the elliptical shaped mirror and substantially reduces the flexing of the mirror as it rotates at high speeds about the rotational axis 32. Although perhaps exaggerated, it is seen that the edge 20 of the mirror portion 24 flexes in the area not reinforced by the central spine 26. This type of edge flexing has become unacceptable with higher mirror speeds.
Therefore, it would be advantageous to provide a torsional hinged mirror that has reduced flexing at the mirror edges.